docs: def _calc_coordinates(self):
          """
          Calculate the coordinates to use when calling an interpolator.
  
          These are needed for `Background2D` and `BackgroundIDW2D`.
  
          Regular-grid interpolators require a 2D array of values.  Some
          require a 2D meshgrid of x and y.  Other require a strictly
          increasing 1D array of the x and y ranges.
          """
  
          # the position coordinates used to initialize an interpolation
          self.y = (self.mesh_yidx * self.box_size[0] +
                    (self.box_size[0] - 1) / 2.)
          self.x = (self.mesh_xidx * self.box_size[1] +
                    (self.box_size[1] - 1) / 2.)
          self.yx = np.column_stack([self.y, self.x])
  
          # the position coordinates used when calling an interpolator
          nx, ny = self.data.shape
          self.data_coords = np.array(list(product(range(ny), range(nx))))